Fixed GUIs not being played after Player reconnection

Author: Sentropic

src/main/java/com/sentropic/guiapi/GUIManager.java

@@ -1,9 +1,11 @@
 package com.sentropic.guiapi;
 
 import com.sentropic.guiapi.gui.GUI;
+import org.bukkit.Bukkit;
 import org.bukkit.entity.Player;
 import org.bukkit.event.EventHandler;
 import org.bukkit.event.Listener;
+import org.bukkit.event.player.PlayerJoinEvent;
 import org.bukkit.event.player.PlayerQuitEvent;
 import org.bukkit.metadata.FixedMetadataValue;
 import org.bukkit.scheduler.BukkitRunnable;
@@ -24,7 +26,10 @@ public class GUIManager implements Listener {
     /**
      * For internal use only. Use {@link GUIAPI#getGUIManager()} to get the instance used by the plugin
      */
-    public GUIManager() { task.runTaskTimer(GUIAPI.getPlugin(), 0, 1); }
+    public GUIManager() {
+        task.runTaskTimer(GUIAPI.getPlugin(), 0, 1);
+        Bukkit.getServer().getOnlinePlayers().forEach(this::createGUI);
+    }
 
     /**
      * For internal use only. Used to clear the stored {@link GUI}s from memory and cancel GUI updating
@@ -43,22 +48,34 @@ void disable() {
      * Gets the {@link GUI} of a given {@link Player}
      *
      * @param player the {@link Player} to get the {@link GUI} for
-     * @return the existing {@link GUI} of the {@link Player}, or a new one if nonexistent
+     * @return the existing {@link GUI} of the {@link Player}
+     * @throws IllegalStateException if the given player is offline
      */
     public GUI getGUI(Player player) {
-        GUI gui;
-        if (player.hasMetadata(METADATA_KEY)) {
-            gui = (GUI) player.getMetadata(METADATA_KEY).get(0).value();
+        if (player.isOnline()) {
+            return (GUI) player.getMetadata(METADATA_KEY).get(0).value();
         } else {
-            gui = new GUI(player);
-            GUIS.add(gui);
-            player.setMetadata(METADATA_KEY, new FixedMetadataValue(GUIAPI.getPlugin(), gui));
+            throw new IllegalStateException();
         }
-        return gui;
     }
 
     @EventHandler
-    public void onPlayerLeave(PlayerQuitEvent event) { GUIS.remove(this.getGUI(event.getPlayer())); }
+    public void onPlayerLeave(PlayerQuitEvent event) {
+        Player player = event.getPlayer();
+        GUIS.remove(this.getGUI(player));
+        player.removeMetadata(METADATA_KEY, GUIAPI.getPlugin());
+    }
+
+    @EventHandler
+    public void onPlayerJoin(PlayerJoinEvent event) {
+        createGUI(event.getPlayer());
+    }
+
+    private void createGUI(Player player) {
+        GUI gui = new GUI(player);
+        player.setMetadata(METADATA_KEY, new FixedMetadataValue(GUIAPI.getPlugin(), gui));
+        GUIS.add(gui);
+    }
 
     private class Task extends BukkitRunnable {
         @Override